Among the devices used as protection against overcurrents, there are circuit breakers which, in case of fault, open the circuit in a mode depending on their tripping characteristics and on the overcurrent value.
The circuit breaker is a mechanical switching device, capable of making, carrying, and breaking currents under normal circuit conditions and also, making and carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short circuit.
Each circuit breaker is equipped with a trip unit which actuates the circuit breaker release mechanism and allows opening on the basis of the current flowing through it.
Two types of trip units are used in low voltage circuit breakers: